The present invention relates to a class of substituted triazolo-pyridazine derivatives and to their use in therapy. More particularly, this invention is concerned with substituted 1,2,4-triazolo[4,3]pyridazine derivatives which are ligands for GABAA receptors and are therefore useful in the therapy of deleterious mental states.
Receptors for the major inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), are divided into two main classes: (1) GABAA receptors, which are members of the ligand-gated ion channel superfamily; and (2) GABAB receptors, which may be members of the G-protein linked receptor superfamily. Since the first cDNAs encoding individual GABAA receptor subunits were cloned the number of known members of the mammalian family has grown to include at least six xcex1 subunits, four xcex2 subunits, three xcex3 subunits, one xcex4 subunit, one xcex5 subunit and two xcfx81 subunits.
Although knowledge of the diversity of the GABAA receptor gene family represents a huge step forward in our understanding of this ligand-gated ion channel, insight into the extent of subtype diversity is still at an early stage. It has been indicated that an xcex1 subunit, a xcex2 subunit and a xcex3 subunit constitute the minimum requirement for forming a fully functional GABAA receptor expressed by transiently transfecting cDNAs into cells. As indicated above, xcex4, xcex5 and xcfx81 subunits also exist, but are present only to a minor extent in GABAA receptor populations.
Studies of receptor size and visualisation by electron microscopy conclude that, like other members of the ligand-gated ion channel family, the native GABAA receptor exists in pentameric form. The selection of at least one xcex1, one xcex2 and one xcex3 subunit from a repertoire of seventeen allows for the possible existence of more than 10,000 pentameric subunit combinations. Moreover, this calculation overlooks the additional permutations that would be possible if the arrangement of subunits around the ion channel had no constraints (i.e. there could be 120 possible variants for a receptor composed of five different subunits).
Receptor subtype assemblies which do exist include, amongst many others, xcex11xcex22xcex32, xcex12xcex22/3xcex32, xcex13xcex2xcex32/3, xcex12xcex2xcex31, xcex15xcex23xcex32/3, xcex16xcex2xcex32, xcex16xcex2xcex4 and xcex14xcex2xcex4. Subtype assemblies containing an xcex11 subunit are present in most areas of the brain and are thought to account for over 40% of GABAA receptors in the rat. Subtype assemblies containing xcex12 and xcex13 subunits respectively are thought to account for about 25% and 17% of GABAA receptors in the rat. Subtype assemblies containing an xcex15 subunit are expressed predominantly in the hippocampus and cortex and are thought to represent about 4% of GABAA receptors in the rat.
A characteristic property of all known GABAA receptors is the presence of a number of modulatory sites, one of which is the benzodiazepine (BZ) binding site. The BZ binding site is the most explored of the GABAA receptor modulatory sites, and is the site through which anxiolytic drugs such as diazepam and temazepam exert their effect. Before the cloning of the GABAA receptor gene family, the benzodiazepine binding site was historically subdivided into two subtypes, BZ1 and BZ2, on the basis of radioligand binding studies. The BZ1 subtype has been shown to be pharmacologically equivalent to a GABAA receptor comprising the xcex11 subunit in combination with a xcex2 subunit and xcex32. This is the most abundant GABAA receptor subtype, and is believed to represent almost half of all GABAA receptors in the brain.
Two other major populations are the xcex12xcex2xcex32 and xcex13xcex2xcex32/3 subtypes. Together these constitute approximately a further 35% of the total GABAA receptor repertoire. Pharmacologically this combination appears to be equivalent to the BZ2 subtype as defined previously by radioligand binding, although the BZ2 subtype may also include certain xcex15-containing subtype assemblies. The physiological role of these subtypes has hitherto been unclear because no sufficiently selective agonists or antagonists were known.
It is now believed that agents acting as BZ agonists at xcex11xcex2xcex32, xcex12xcex2xcex32 or xcex13xcex2xcex32 subunits will possess desirable anxiolytic properties. Compounds which are modulators of the benzodiazepine binding site of the GABAA receptor by acting as BZ agonists are referred to hereinafter as xe2x80x9cGABAA receptor agonistsxe2x80x9d. The xcex11-selective GABAA receptor agonists alpidem and zolpidem are clinically prescribed as hypnotic agents, suggesting that at least some of the sedation associated with known anxiolytic drugs which act at the BZ1 binding site is mediated through GABAA receptors containing the xcex11 subunit. Accordingly, it is considered that GABAA receptor agonists which interact more favourably with the xcex12 and/or xcex13 subunit than with xcex11 will be effective in the treatment of anxiety with a reduced propensity to cause sedation. Also, agents which are antagonists or inverse agonists at xcex11 might be employed to reverse sedation or hypnosis caused by xcex11 agonists.
The compounds of the present invention, being selective ligands for GABAA receptors, are therefore of use in the treatment and/or prevention of a variety of disorders of the central nervous system. Such disorders include anxiety disorders, such as panic disorder with or without agoraphobia, agoraphobia without history of panic disorder, animal and other phobias including social phobias, obsessive-compulsive disorder, stress disorders including post-traumatic and acute stress disorder, and generalized or substance-induced anxiety disorder; neuroses; convulsions; migraine; depressive or bipolar disorders, for example single-episode or recurrent major depressive disorder, dysthymic disorder, bipolar I and bipolar II manic disorders, and cyclothymic disoder; psychotic disorders including schizophrenia; neurodegeneration arising from cerebral ischemia; attention deficit hyperactivity disorder; and disorders of circadian rhythm, e.g. in subjects suffering from the effects of jet lag or shift work.
Further disorders for which selective ligands for GABAA receptors may be of benefit include pain and nociception; emesis, including acute, delayed and anticipatory emesis, in particular emesis induced by chemotherapy or radiation, as well as post-operative nausea and vomiting; eating disorders including anorexia nervosa and bulimia nervosa; premenstrual syndrome; muscle spasm or spasticity, e.g. in paraplegic patients; and hearing loss. Selective ligands for GABAA receptors may also be effective as pre-medication prior to anaesthesia or minor procedures such as endoscopy, including gastric endoscopy.
In DE-A-2741763, and in U.S. Pat. Nos. 4,260,755, 4,260,756 and 4,654,343, are described various classes of 1,2,4-triazolo[4,3-b]pyridazine derivatives which are alleged to be useful as anxiolytic agents. The compounds described in DE-A-2741763 and in U.S. Pat. Nos. 4,260,755 and 4,654,343 possess a phenyl substituent at the 6-position of the triazolo-pyridazine ring system. The compounds described in U.S. Pat. No. 4,260,756, meanwhile, possess a heteroaryl moiety at the 6- or 8-position. In none of these publications, however, is there any disclosure or suggestion of 1,2,4-triazolo[4,3-b]pyridazine derivatives wherein the substituent at the 6-position is attached through a directly linked nitrogen atom.
EP-A-0085840 and EP-A-0134946 describe related series 1,2,4-triazolo[3,4-xcex1]phthalazine derivatives which are stated to possess antianxiety activity. However, there is no disclosure nor any suggestion in either of these publications of replacing the benzo moiety of the triazolo-phthalazine ring system with any other functionality.
The present invention provides a class of triazolo-pyridazine derivatives which possess desirable binding properties at various GABAA receptor subtypes. The compounds in accordance with the present invention have good affinity as ligands for the xcex12 and/or xcex13 subunit of the human GABAA receptor. The compounds of this invention may interact more favourably with the xcex12 and/or xcex13 subunit than with the xcex11 subunit. Desirably, the compounds of the invention will exhibit functional selectivity in terms of a selective efficacy for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit.
The compounds of the present invention are GABAA receptor subtype ligands having a binding affinity (Ki) for the xcex12 and/or xcex13 subunit, as measured in the assay described hereinbelow, of 100 nM or less, typically of 50 nM or less, and ideally of 10 nM or less. The compounds in accordance with this invention may possess at least a 2-fold, suitably at least a 5-fold, and advantageously at least a 10-fold, selective affinity for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit. However, compounds which are not selective in terms of their binding affinity for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit are also encompassed within the scope of the present invention; such compounds will desirably exhibit functional selectivity in terms of a selective efficacy for the xcex12 and/or xcex13 subunit relative to the xcex11 subunit.
The present invention provides a compound of formula I, or a salt or prodrug thereof: 
wherein
Y represents hydrogen or C1-6 alkyl;
Z represents C1-6 alkyl, C3-7 cycloalkyl, C4-7 cycloalkenyl, aryl, C3-7 heterocycloalkyl, heteroaryl or di(C1-6)alkylamino, any of which groups may be optionally substituted;
R1 represents C3-7 cycloalkyl, phenyl, furyl, thienyl or pyridinyl, any of which groups may be optionally substituted;
R2 represents hydrogen or C1-6 alkyl; and
R3 represents hydrogen; or C1-6 alkyl, C3-7 cycloalkyl(C1-6)alkyl, propargyl, aryl(C1-6)alkyl, C3-7 heterocycloalkyl(C1-6)alkyl or heteroaryl(C1-6)alkyl, any of which groups may be optionally substituted; or
R2 and R3 are taken together with the intervening nitrogen atom to form a ring of formula (a) or (b): 
wherein
X represents oxygen, sulphur or Nxe2x80x94R4, in which
R4 represents hydrogen or C1-6 alkyl.
The groups Z, R1 and R3 may be unsubstituted, or substituted by one or more, suitably by one or two, substituents. In general, the groups Z, R1 and R3 will be unsubstituted or monosubstituted. Examples of optional substituents on the groups Z, R1 and R3 include C1-6 alkyl, aryl(C1-6)alkyl, pyridyl(C1-6)alkyl, halogen, halo(C1-6)alkyl, cyano, cyano(C1-6)alkyl, oxo, hydroxy, hydroxymethyl, C1-6 alkoxy, C3-7 cycloalkyl(C1-6)alkoxy, C3-7 cycloalkoxy, amino, C1-6 alkylamino, di(C1-6)alkylamino, amino(C1-6)alkyl, di(C1-6)alkylamino(C1-6)alkyl, di(C1-6)alkylaminocarbonyl(C1-6)alkyl, N-(C1-6)alkylpiperidinyl, pyrrolidinyl(C1-6)alkyl, piperazinyl(C1-6)alkyl, morpholinyl(C1-6)alkyl, di(C1-6)alkylmorpholinyl(C1-6)alkyl and imidazolyl(C1-6)alkyl. Representative substituents include C1-6 alkyl, halogen, oxo, C1-6 alkoxy and di(C1-6)alkylamino.
As used herein, the expression xe2x80x9cC1-6 alkylxe2x80x9d includes methyl and ethyl groups, and straight-chained or branched propyl, butyl, pentyl and hexyl groups. Particular alkyl groups are methyl, ethyl, n-propyl, isopropyl, tert-butyl and 1,1-dimethylpropyl. Derived expressions such as xe2x80x9cC1-6alkoxyxe2x80x9d are to be construed accordingly.
Typical C3-7 cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl.
The expression xe2x80x9cC3-7 cycloalkyl(C1-6)alkylxe2x80x9d as used herein includes cyclopropylmethyl, cyclobutylmethyl, cyclopentylmethyl and cyclohexylmethyl.
Typical C4-7 cycloalkenyl groups include cyclobutenyl, cyclopentenyl and cyclohexenyl.
Typical aryl groups include phenyl and naphthyl, preferably phenyl.
The expression xe2x80x9caryl(C1-6)alkylxe2x80x9d as used herein includes benzyl, phenylethyl, phenylpropyl and naphthylmethyl.
Suitable heterocycloalkyl groups include azetidinyl, pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl and thiomorpholinyl groups.
The expression xe2x80x9cC3-7 cycloalkyl(C1-6)alkylxe2x80x9d as used herein includes pyrrolidinylethyl, piperazinylethyl, morpholinylethyl, pyrrolidinylpropyl and morpholinylpropyl.
Suitable heteroaryl groups include pyridinyl, quinolinyl, isoquinolinyl, pyridazinyl, pyrimidinyl, pyrazinyl, quinoxalinyl, furyl, benzofuryl, dibenzofuryl, thienyl, benzthienyl, pyrrolyl, indolyl, pyrazolyl, indazolyl, oxazolyl, isoxazolyl, thiazolyl, isothiazolyl, imidazolyl, benzimidazolyl, oxadiazolyl, thiadiazolyl, triazolyl and tetrazolyl groups.
The expression xe2x80x9cheteroaryl(C1-6)alkylxe2x80x9d as used herein includes furylmethyl, furylethyl, thienylmethyl, thienylethyl, pyrazolylmethyl, oxazolylmethyl, oxazolylethyl, isoxazolylmethyl, thiazolylmethyl, thiazolylethyl, imidazolylmethyl, imidazolylethyl, imidazolylpropyl, benzimidazolylmethyl, oxadiazolylmethyl, oxadiazolylethyl, thiadiazolylmethyl, thiadiazolylethyl, triazolylmethyl, triazolylethyl, tetrazolylmethyl, tetrazolylethyl, pyridinylmethyl, pyridinylethyl, pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl.
The term xe2x80x9chalogenxe2x80x9d as used herein includes fluorine, chlorine, bromine and iodine, especially fluorine or chlorine.
For use in medicine, the salts of the compounds of formula I will be pharmaceutically acceptable salts. Other salts may, however, be useful in the preparation of the compounds according to the invention or of their pharmaceutically acceptable salts. Suitable pharmaceutically acceptable salts of the compounds of this invention include acid addition salts which may, for example, be formed by mixing a solution of the compound according to the invention with a solution of a pharmaceutically acceptable acid such as hydrochloric acid, sulphuric acid, methanesulphonic acid, fumaric acid, maleic acid, succinic acid, acetic acid, benzoic acid, oxalic acid, citric acid, tartaric acid, carbonic acid or phosphoric acid. Furthermore, where the compounds of the invention carry an acidic moeity, suitable pharmaceutically acceptable salts thereof may include alkali metal salts, e.g. sodium or potassium salts; alkaline earth metal salts, e.g. calcium or magnesium salts; and salts formed with suitable organic ligands, e.g. quaternary ammonium salts.
The present invention includes within its scope prodrugs of the compounds of formula I above. In general, such prodrugs will be functional derivatives of the compounds of formula I which are readily convertible in vivo into the required compound of formula I. Conventional procedures for the selection and preparation of suitable prodrug derivatives are described, for example, in Design of Prodrugs, ed. H. Bundgaard, Elsevier, 1985.
Where the compounds according to the invention have at least one asymmetric centre, they may accordingly exist as enantiomers. Where the compounds according to the invention possess two or more asymmetric centres, they may additionally exist as diasteroisomers. It is to be understood that all such isomers and mixtures thereof in any proportion are encompassed within the scope of the present invention.
Suitably, Y represents hydrogen or methyl, especially hydrogen.
Examples of suitable values for the substituent Z include methyl, ethyl, isopropyl, tert-butyl, 1,1-dimethylpropyl, methyl-cyclopropyl, cyclobutyl, methyl-cyclobutyl, cyclopentyl, methyl-cyclopentyl, cyclohexyl, cyclobutenyl, phenyl, pyrrolidinyl, methyl-pyrrolidinyl, piperidinyl, morpholinyl, thiomorpholinyl, pyridinyl, furyl, thienyl, thienyl, chloro-thienyl and diethylamino.
In a particular embodiment, the substituent Z represents C3-7 cycloalkyl, either unsubstituted or substituted C1-6 alkyl, especially methyl. Favourably, Z represents cyclobutyl.
Examples of typical optional substituents on the group R1 include methyl, fluoro and methoxy.
Representative values of R1 include cyclopropyl, phenyl, methylphenyl, fluorophenyl, difluorophenyl, methoxyphenyl, furyl, thienyl, methyl-thienyl and pyridinyl. Suitably, R1 may represent unsubstituted or monosubstituted phenyl. More particularly, R1 represents phenyl.
Suitabley, R2 represents hydrogen or methyl.
Suitable values for the substituent R3 in the compounds according to the invention include hydrogen; and methyl, ethyl, propyl, butyl, cyclohexylmethyl, propargyl, benzyl, pyrrolidiniylethyl, piperazinylethyl, morpholinylethyl, pyrrolidinylpropyl, morpholinylpropyl, pyrazolylmethyl, isoxazolylmethyl, thiazolylmethyl, thiazolylethyl, imidazolylmethyl, imidazolylpropyl, benzimidazolylmethyl, oxadiazolylmethyl, triazolylmethyl, tetrazolylmethyl, pyridinylmethyl, pyridinylethyl, pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoaxalinylmethyl, any of which groups may be optionally substituted by one or more substituents. Typical values of R3 include hydrogen; and methyl, ethyl, propyl, pyrrolidinylethyl, piperazinylethyl, morpholinylethyl, pyrrolidinylpropyl, morpholinylpropyl, imidazolylpropyl, triazolylmethyl, pyridinylmethyl and pyridinylethyl, any of which groups may be optionally substituted by one or more substituents.
Examples of suitable optional substituents on the group R3 include C1-6 alkyl, aryl(C1-6)alkyl, pyridyl(C1-6)alkyl, halogen, halo(C1-6)alkyl, cyano, cyano(C1-6)alkyl, oxo, hydroxymethyl, C1-6alkoxy, C3-7 cycloalkyl(C1-6)alkoxy, di(C1-6)alkylamino, amino(C1-6)alkyl, di(C1-6)alkylamino(C1-6)alkyl, di(C1-6)alkylaminocarbonyl(C1-6)alkyl, N-(C1-6)alkylpiperidinyl, pyrrolidinyl(C1-6)alkyl, piperazinyl(C1-6)alkyl, morpholinyl(C1-6)alkyl and di(C1-6)alkylmorpholinyl(C1-6)alkyl, Typical substituents include C1-6 alkyl, aryl(C1-6)alkyl, halogen, cyano, oxo hydroxymethyl, C1-6 alkoxy, C3-7 cycloalkyl(C1-6)alkoxy and di(C1-6)alkylamino, especially C1-6 alkyl, oxo, C1-6 alkoxy or di(C1-6)alkylamino.
Specific illustrations of particular substituents on the group R3 include methyl, ethyl, n-propyl, benzyl, pyridinylmethyl, chloro, chloromethyl, cyano, cyanomethyl, oxo, hydroxymethyl, methoxy, ethoxy, cyclopropylmethoxy, dimethylamino, dimethylaminomethyl, aminoethyl, dimethylaminoethyl, dimethylaminocarbonylmethyl, N-methylpiperidinyl, pyrrolidinylethyl, piperazinylethyl, morpholinylmethyl and dimethylmorpholinylmethyl, especially methyl, oxo, methoxy or dimethylamino.
Representative values of R3 include hydrogen, methyl, cyanomethyl, methoxyethyl, dimethylaminoethyl, dimethylaminopropyl, hydroxybutyl, hydroxymethyl-cyclohexylmethyl, propargyl, dimethylaminomethyl-propargyl, dimethylmorphinolinylmethyl-propargyl, cyanobenzyl, hydroxymethyl-benzyl, methyl-pyrrolidinylethyl, piperazinylethyl, morpholinylethyl, pyrrolidinylpropyl, pyrrolidinonylpropyl, morpholinylpropyl, pyrazolylmethyl, dimethyl-pyrazolylmethyl, methyl-isoxazolylmethyl, thiazolylmethyl, methyl-thiazolylmethyl, ethyl-thiazolylmethyl, methyl-thiazolylethyl, imidazolylmethyl, methyl-imidazolylmethyl, ethyl-imidazolylmethyl, benzyl-imidazolylmethyl, imidazolylpropyl, benzimadozolylmethyl, methyl-oxadiazolylmethyl, triazolylmethyl, methyl-triazolylmethyl, propyl-triazolylmethyl, benzyl-triazolylmethyl, pyridinylmethyl-triazolylmethyl, cyanomethyl-triazolylmethyl, dimethylaminomethyl-triazolylmethyl, aminoethyl-triazolylmethyl, dimethylaminoethyl-triazolylmethyl, dimethylaminocarbonylmethyl-triazolylmethyl, N-methylpiperidinyl-triazolylmethyl, pyrrolidinylethyl-triazolylmethyl, piperazinylethyl-triazolylmethyl, morpholinylethyl-triazolylmethyl, methyl-tetrazolylmethyl, pyridinylmethyl, methyl-pyridinylmethyl, dimethyl-pyridinylmethyl, ethoxy-pyridinylmethyl, cyclopropylmethoxy-pyridinylmethyl, pyridinylethyl, pyridazinylmethyl, chloro-pyridazinylmethyl, pyrimidinylmethyl, pyrazinylmethyl, quinolinylmethyl, isoquinolinylmethyl and quinoxalinylmethyl.
Particular values of R3 include hydrogen, methyl, methoxyethyl, dimethylaminoethyl, dimethylaminopropyl, methyl-pyrrolidinylethyl, piperazinylethyl, morpholinylethyl, pyrrolidinylpropyl, pyrrolidinonylpropyl, morpholinylpropyl, imidazolylpropyl, methyl-triazolylmethyl, pyridinylmethyl and pyridinylethyl.
A favoured value of R3 is methyl-triazolylmethyl.
Suitably, R4 represents hydrogen or methyl.
Where R2 and R3 are taken together with the intervening nitrogen atom to form a ring of formula (a) as depicted above, the moiety X suitably represents oxygen.
A particular sub-class of compounds according to the invention is represented by the compounds of formula IIA, and salts and prodrugs thereof: 
wherein
Y1 represents hydrogen or methyl;
Z, R1 and R2 are as defined with reference to formula I above;
M is 1, 2 or 3, preferably 1; and
R13 represents aryl or heteroaryl, either of which groups may be optionally substituted by one or more substituents.
Suitably, Y1 represents hydrogen.
Examples of typical substituents on the group R13 include C1-6 alkyl, aryl(C1-6)alkyl, pyridyl(C1-6)alkyl, halogen, cyano, cyano(C1-6)alkyl, hydroxymethyl, C1-6 alkoxy, C3-7 cycloalkyl(C1-6)alkoxy, di(C1-6)alkylamino(C1-6)alkyl, amino(C1-6)alkyl, di(C1-6)alkylaminocarbonyl(C1-6)alkyl, N-(C1-6)alkylpiperdinyl, pyrrolidinyl(C1-6)alkyl, piperazinyl(C1-6)alkyl and morpholinyl(C1-6)alkyl, especially C1-6 alkyl or C1-6 alkoxy.
Illustrative values of specific substituents on the group R13 include methyl, ethyl, n-propyl, benzyl, pyridinylmethyl, chloro, cyano, cyanomethyl, hydroxymethyl, methoxy, ethoxy, cyclopropylmethoxy, dimethylaminomethyl, aminoethyl, dimethylaminoethyl, dimethylaminocarbonylmethyl, N-methylpiperidinyl, pyrrolidinylethyl, piperazinylethyl and morpholinylmethyl, especially methyl.
Particular values of R13 include cyanophenyl, hydroxymethyl-phenyl, pyrazolyl, dimethyl-pyrazolyl, methyl-isoxazolyl, thiazolyl, methyl-thiazolyl, ethyl-thiazolyl, imidazolyl, methyl-imidazolyl, ethyl-imidazolyl, benzyl-imidazolyl, benzimidazolyl, methyl-oxadiazolyl, triazolyl, methyl-triazolyl, propyl-triazolyl, benzyl-triazolyl, pyridinylmethyl-triazolyl, cyanomethyl-triazolyl, dimethylaminomethyl-triazolyl, aminoethyl-triazolyl, dimethylaminoethyl-triazolyl, dimethylaminocarbonylmethyl-triazolyl, N-methylpiperidinyl-triazolyl, pyrrolidinylethyl-triazolyl, piperazinylethyl-triazolyl, morpholinylethyl-triazolyl, methyl-tetrazolyl, pyridinyl, methyl-pyridinyl, dimethyl-pyridinyl, ethoxy-pyridinyl, cyclopropylmethoxy-pyridinyl, pyridazinyl, chloro-pyrdiazinyl, pyrimidinyl, pyrazinyl, quinolinyl, isoquinolinyl and quinoxoaliny.
Specific values of R13 include imidazolyl, methyl-triazolyl and pyridinyl.
A favoured value of R13 is methyl-triazolyl.
A particular subset of the compounds of formula IIA above is represented by the compounds of formula IIB, and pharmaceutically acceptable salts thereof: 
wherein
R1 and R2 are as defined with reference to formula I above;
Q represents the residue of a cyclopropyl, cyclobutyl, cyclopentyl or cyclohexyl ring;
R5 represents hydrogen or methyl; and
R6 represents hydrogen or methyl.
In relation to formula IIB above, R1 suitable represents phenyl.
In a favoured embodiment, Q suitably represents the residue of a cyclobutyl ring.
Suitably, R5 represents hydrogen.
Suitably, R6 represents methyl.
Specific compounds within the scope of the present invention include:
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-(1-methyl-1H-1,2,4-triazol-3-ylmethyl)amine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N,N-dimethylamine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-methyl-N-(1-methyl-1H-1,2,4-triazol-3-ylmethyl)amine;
7-cyclobutyl-6-(5,6-dihydro-8H-1,2,4-triazolo[1,5-xcex1]pyrazin-7-yl)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine;
7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-ylamine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-methylamine;
7-cyclobutyl-6-(morpholin-4-yl)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-(2-methoxyethyl)amine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-(pyridin-2-ylmethyl)amine;
Nxe2x80x2-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N,N-dimethylethane-1,2-diamine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[3-(morpholin-4-yl)propyl]amine;
(xc2x1)-N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[2-(1-methylpyrrolidin-2-yl)ethyl]amine;
Nxe2x80x2-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N,N-dimethylpropane-1,3-diamine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[3-(imidazol-1-yl)propyl]amine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[3-(pyrrolidin-1-yl)propyl]amine;
1-[3-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-ylamino)-propyl]pyrrolidin-2-one;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[2-(piperazin-1-yl)ethyl]amine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[2-(morpholin-4-yl)ethyl]amine;
N-(7-cyclobutyl-3-phenyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)-N-[2-(pyridin-2-yl)ethyl]amine;
and salts and prodrugs thereof.
Also provided by the present invention is a method for the treatment and/or prevention of anxiety which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined above or a pharmaceutically acceptable salt thereof or a prodrug thereof.
Further provided by the present invention is a method for the treatment and/or prevention of convulsions (e.g. in a patient suffering from epilepsy or a related disorder) which comprises administering to a patient in need of such treatment an effective amount of a compound of formula I as defined above or a pharmaceutically acceptable salt thereof or a prodrug thereof.
The binding affinity (Ki) of the compounds according to the present invention for the xcex13 subunit of the human GABAA receptor is conveniently as measured in the assay described hereinbelow. The xcex13 subunit binding affinity (Ki) of the compounds according to the invention is ideally 10 nM or less, preferably 2 nM or less, and more preferably 1 nM or less.
The compounds according to the present invention will ideally elicit at least a 40%, preferably at least a 50%, and more preferably at least a 60%, potentiation of the GABA EC20 response in stably transfected recombinant cell lines expressing the xcex13 subunit of the human GABAA receptor. Moreover, the compounds of the invention will ideally elicit at most a 30%, preferably at most a 20%, and more preferably at most a 10%, potentiation of the GABA EC20 response in stably transfected recombinant cell lines expressing the xcex11 subunit of the human GABAA receptor.
The potentiation of the GABA EC20 response in stably transfected cell lines expressing the xcex13 and xcex11 subunits of the human GABAA receptor can conveniently be measured by procedures analogous to the protocol described in Wafford et al., Mol. Pharmacol., 1996, 50, 670-678. The procedure will suitably be carried out utilising cultures of stably transfected eukaryotic cells, typically of stably transfected mouse Ltk fibroblast cells.
The compounds according to the present invention exhibit anxiolytic activity, as may be demonstrated by a positive response in the elevated plus maze and conditioned suppression of drinking tests (cf. Dawson et al., Psychopharmacology, 1995, 121, 109-117). Moreover, the compounds of the invention are substantially non-sedating, as may be confirmed by an appropriate result obtained from the response sensitivity (chain-pulling) test (cf. Bayley et al., J. Psychopharmacol., 1996, 10, 206-213).
The compounds according to the present invention may also exhibit anticonvulsant activity. This can be demonstrated by the ability to block pentylenetetrazole-induced seizures in rats and mice, following a protocol analogous to that described by Bristow et al. in J. Pharmacol. Exp. Ther., 1996, 279, 492-501.
In order to elicit their behavioural effects, the compounds of the invention will be brain-penetrant; in other words, these compounds will be capable of crossing the so-called xe2x80x9cblood-brain barrierxe2x80x9d. Preferably, the compounds of the invention will be capable of exerting their beneficial therapeutic action following administration by the oral route.
The invention also provides pharmaceutical compositions comprising one or more compounds of the invention in association with a pharmaceutically acceptable carrier. Preferably these compositions are in unit dosage forms such as tablets, pills, capsules, powders, granules, sterile parenteral solutions or suspensions, metered aerosol or liquid sprays, drops, ampoules, auto-injector devices or suppositories; for oral, parenteral, intranasal, sublingual or rectal administration, or for administration by inhalation or insufflation. For preparing solid compositions such as tablets, the principal active ingredient is mixed with a pharmaceutical carrier, e.g. conventional tableting ingredients such as corn starch, lactose, sucrose, sorbitol, talc, stearic acid, magnesium stearate, dicalcium phosphate or gums, and other pharmaceutical diluents, e.g. water, to form a solid preformulation composition containing a homogeneous mixture of a compound of the present invention, or a pharmaceutically acceptable salt thereof. When referring to these preformulation compositions as homogeneous, it is meant that the active ingredient is dispersed evenly throughout the composition so that the composition may be readily subdivided into equally effective unit dosage forms such as tablets, pills and capsules. This solid preformulation composition is then subdivided into unit dosage forms of the type described above containing from 0.1 to about 500 mg of the active ingredient of the present invention. Typical unit dosage forms contain from 1 to 100 mg, for example, 1, 2, 5, 10, 25, 50 or 100 mg, of the active ingredient. The tablets or pills of the novel composition can be coated or otherwise compounded to provide a dosage form affording the advantage of prolonged action. For dosage component, the latter being in the form of an envelope over the former. The two components can be separated by an enteric layer which serves to resist disintegration in the stomach and permits the inner component to pass intact into the duodenum or to be delayed in release. A variety of materials can be used for such enteric layers or coatings, such materials including a number of polymeric acids and mixtures of polymeric acids with such materials as shellac, cetyl alcohol and cellulose acetate.
The liquid forms in which the novel compositions of the present invention may be incorporated for administration orally or by injection include aqueous solutions, suitably flavoured syrups, aqueous or oil suspensions, and flavoured emulsions with edible oils such as cottonseed oil, sesame oil, coconut oil or peanut oil, as well as elixirs and similar pharmaceutical vehicles. Suitable dispersing or suspending agents for aqueous suspensions include synthetic and natural gums such as tragacanth, acacia, alginate, dextran, sodium carboxymethylcellulose, methylcellulose, polyvinyl-pyrrolidone or gelatin.
In the treatment of anxiety, a suitable dosage level is about 0.01 to 250 mg/kg per day, preferably about 0.05 to 100 mg/kg per day, and especially about 0.05 to 5 mg/kg per day. The compounds may be administered on a regimen of 1 to 4 times per day.
The compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula III with a compound of formula IV: 
wherein Y, Z, R1, R2 and R3 are as defined above; and L1 represents a suitable leaving group.
The leaving group L1 is typically a halogen atom, especially chloro.
The reaction between compounds III and IV is conveniently effected by heating the reactants together in a sealed tube, optionally in a suitable solvent such as N,N-dimethylformamide, methanol or 1,4-dioxane, and optionally in the presence of a base such as triethylamine. Where R2 and R3 together with the intervening nitrogen atom represent a ring of formula (b) as defined above, the reaction between compounds III and IV may conveniently be effected by stirring the reactants in an inert solvent, e.g. dimethyl sulphoxide, at an elevated temperature, typically in the region of 150xc2x0 C.
The intermediates of formula III above may be prepared by reacting a compound of formula V with a substantially equimolar amount of a hydrazine derivative of formula VI: 
wherein Y, Z, R1 and L1 are as defined above, and L2 represents a suitable leaving group; followed, if necessary, by separation of the resulting mixture of isomers by conventional means.
The leaving group L2 is typically a halogen atom, especially chloro. In the intermediates of formula V, the leaving groups L1 and L2 may be the same or different, but are suitably the same, preferably both chloro.
The reaction between compounds V and VI is conveniently effected by heating the reactants in the presence of a proton source such as triethylamine hydrochloride, typically at reflux in an inert solvent such as xylene or 1,4-dioxane.
Where Y and Z are different, the reaction between compounds V and VI will, as indicated above, usually give rise to a mixture of isomeric products depending upon whether the hydrazine derivative VI displaces the leaving group L1 or L2. Thus, in addition to the required product of formula III, the isomeric compound wherein the Y and Z moieties are reversed will usually be obtained to some extent. For this reason it will generally be necessary to separate the resulting mixture of isomers by conventional methods such as chromatography.
In another procedure, the compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula Zxe2x80x94CO2H with a compound of formula VII: 
wherein Y, Z, R1, R2 and R3 are as defined above; in the presence of silver nitrate and ammonium persulphate.
The reaction is conveniently carried out under acidic conditions in a suitable solvent, for example using sulphuric acid in water or aqueous acetonitrile, typically at an elevated temperature.
The intermediates of formula VII correspond to the compounds of formula I as defined above wherein Z is hydrogen, and they may therefore be prepared by methods analogous to those described above for preparing the corresponding compounds of formula I.
In a further procedure, the compounds of formula I as defined above may be prepared by a process which comprises reacting a compound of formula VIII with a compound of formula IX: 
wherein Y, Z, R1, R2 and R3 are as defined above, M represents xe2x80x94B(OH)2 or xe2x80x94Sn(Alk)3 in which Alk represents a C1-6 alkyl group, typically n-butyl, and L4 represents a suitable leaving group; in the presence of a transition metal catalyst.
The leaving group L4 is suitably a halogen atom, e.g. bromo.
A suitable transition metal catalyst of use in the reaction between compounds VIII and IX comprises dichlorobis(triphenylphosphine)-palladium(II) or tetrakis(triphenylphosphine)palladium(0).
The reaction between compounds VIII and IX is conveniently effected in an inert solvent such as N,N-dimethylformamide, typically at an elevated temperature.
The intermediates of formula VIII may be prepared by reacting a compound of formula IV as defined above with a compound of formula X: 
wherein Y, Z, L1 and L4 are as defined above; under conditions analogous to those described above for the reaction between compounds III and IV.
The intermediates of formula V above may be prepared by reacting a compound of formula Zxe2x80x94CO2H with a compound of formula XI: 
wherein Y, Z, L1 and L2 are as defined above; in the presence of silver nitrate and ammonium persulphate; under conditions analogous to those described above for the reaction between Zxe2x80x94CO2H and compound VII.
Where they are not commercially available, the starting materials of formula IV, VI, IX, X, and XI may be prepared by methods analogous to those described in the accompanying Examples, or by standard methods well known from the art.
It will be understood that any compound of formula I initially obtained from any of the above processes may, where appropriate, subsequently be elaborated into a further compound of formula I by techniques known from the art. For example, a compound of formula I initially obtained wherein R3 is unsubstituted may be converted into a corresponding compound wherein R3 is substituted, typically be standard alkylation procedures, for example by treatment with a haloalkyl derivative in the presence of sodium hydride and N,N-dimethylformamide, or with a hydroxyalkyl derivative in the presence of triphenylphosphine and diethyl azodicarboxylate. Furthermore, a compound of formula I initially obtained wherein R3 represents cyano(C1-6)alkyl may be converted into the corresponding 3-substituted 1,2,4-triazol-5-yl(C1-6)alkyl analogue by treatment with the appropriate acyl hydrazine derivative in the presence of a base such as sodium methoxide. Similarly, a compound of formula I initially obtained wherein R3 represents an optionally substituted propargyl moiety may be converted into the corresponding 1,2,3-triazolylmethyl analogue by treatment with azide anion. A compound of formula I initially obtained wherein the R3 substituent is substituted by a halogen atom, e.g. chloro, may be converted into the corresponding compound wherein the R3 substituent is substituted by a di(C1-6)alkylamino moiety by treatment with the appropriate di(C1-6)alkylamine, typically with heating in a solvent such as 1,4-dioxane in a sealed tube.
Where the above-described processes for the preparation of the compounds according to the invention give rise to mixtures of stereoisomers, these isomers may be separated by conventional techniques such as preparative chromatography. The novel compounds may be prepared in racemic form, or individual enantiomers may be prepared either by enantiospecific synthesis or by resolution. The novel compounds may, for example, be resolved into their component enantiomers by standard techniques such as preparative HPLC, or the formation of diastereomeric pairs by salt formation with an optically active acid, such as (xe2x88x92)-di-p-toluoyl-d-tartaric acid and/or (+)-di-p-toluoyl-l-tartaric acid, followed by fractional crystallization and regeneration of the free base. The novel compounds may also be resolved by formation of diastereomeric esters or amides, followed by chromatographic separation and removal of the chiral auxiliary.
During any of the above synthetic sequences it may be necessary and/or desirable to protect sensitive or reactive groups on any of the molecules concerned. This may be achieved by means of conventional protecting groups, such as those described in Protective Groups in Organic Chemistry, ed. J. F. W. McOmie, Plenum Press, 1973; and T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, John Wiley and Sons, 1991. The protecting groups may be removed at a convenient subsequent stage using methods known from the art.
The following Examples illustrate the preparation of compounds according to the invention.
The compounds in accordance with this invention potently inhibit the binding of [3H]-flumazenil to the benzodiazepine binding site of human GABAA receptors containing the xcex12 or xcex13 subunit stably expressed in Ltk cells.
Reagents
Phosphate buffered saline (PBS).
Assay buffer: 10 mM KH2PO4, 100 mM KCl, pH 7.4 at room temperature.
[3H]-Flumazenil (18 nM for xcex11xcex23xcex32 cells; 18 nM for xcex12xcex23xcex32 cells; 10 nM for xcex13xcex23xcex32 cells) in assay buffer.
Flunitrazepam 100 xcexcM in assay buffer.
Cells resuspended in assay buffer (1 tray to 10 ml).
Haversting Cells
Supernatant is removed from cells. PBS (approximately 20 ml) is added. The cells are scraped and placed in a 50 ml centrifuge tube. The procedure is repeated with a further 10 ml of PBS to ensure that most of the cells are removed. The cells are pelleted by centrifuging for 20 min at 3000 rpm in a benchtop centrifuge, and then frozen if desired. The pellets are resuspended in 10 ml of buffer per tray (25 cmxc3x9725 cm) of cells.
Assay
Can be carried out in deep 96-well plates or in tubes. Each tube contains:
300 xcexcl of assay buffer.
50 xcexcl of [3H]-flumazenil (final concentration for xcex11xcex23xcex32: 1.8 nM; for xcex12xcex23xcex32: 1.8 nM; for xcex13xcex23xcex32: 1.0 nM).
50 xcexcl of buffer or solvent carrier (e.g. 10% DMSO) if compounds are dissolved in 10% DMSO (total); test compound or flunitrazepam (to determine non-specific binding), 10 xcexcM final concentration.
100 xcexcl of cells.
Assays are incubated for 1 hour at 40xc2x0 C., then filtered using either a Tomtec or Brandel cell harvester onto GF/B filters followed by 3xc3x973 ml washes with ice cold assay buffer. Filters are dried and counted by liquid scintillation counting. Expected values for total binding are 3000-4000 dpm for total counts and less than 200 dpm for non-specific binding if using liquid scintillation counting, or 1500-2000 dpm for total counts and less than 200 dpm for non-specific binding if counting with meltilex solid scintillant. Binding parameters are determined by non-linear least squares regression analysis, from which the inhibition constant Ki can be calculated for each test compound.
The compounds of the accompanying Examples were tested in the above assay, and all were found to possess a Ki value for displacement of [3H]-flumazenil from the xcex12 and/or xcex13 subunit of the human GABAA receptor of 100 nM or less.